Digest should be an IO

[straight-shoota]

The Digest class behaves as a stream writer, so it would make sense to have it inherit IO.

This would allow using Digest implementations as data sink in an IO composition.
For example, in combination with IO::MultiWriter or IO::DupReader (proposed in #14792), it would be trivial to write / read to an IO, while simultaneously calculating a digest of the data.

For example, the following program prints some data to an IO, and calculates the CRC32 on the fly:

require "digest/crc32"

crc = Digest::CRC32.new
io = IO::MultiWriter.new(STDOUT, crc)

io << "Hello "
io << "Crystal!\n"

p crc.final

The Digest#update(data : Bytes) method is essentially the same as IO#write(slice : Bytes), making this similarity very obvious.

Digest even implements << which can be used in a very similar way as in IO. It's not identical, though. Digest#<< is just an alias for #update, while IO#<< implements string concatenation (it calls data.to_s(self), not write(data)).
This would probably be the most difficult problem for integrating Digest with IO, due to the different semantics of #<<.
I don't think there are any other incompatibilities.

[ysbaddaden]

Excellent use-case!